99 7.3 Powerstroke, 3.73s. Tow everything from a car hauler to 10,000-pound travel trailer in overdrive. Rule of thumb, if you are under 55 mph lock it out or if you are climbing a hill lock it out. Another rule of thumb, stay over 75 mph and don’t worry about shifting out of overdrive. I think the key is don’t let the trans “hunt” or shift back and forth between 3rd and 4th.